FIG. 22 is a cross-sectional view of conventional acoustic wave device 101. Acoustic wave device 101 includes piezoelectric substrate 102, IDT electrode 103A formed on piezoelectric substrate 102, and dielectric film 104 formed above substrate 102. IDT electrode 103A includes electrode fingers 103 that excite a Rayleigh wave as a main acoustic wave. Dielectric film 104 covers electrode fingers 103 of IDT electrode 103A.
Dielectric film 104 is made of silicon oxide. The temperature coefficient of frequency (TCF) of silicon oxide has a sign opposite to that of piezoelectric substrate 102, so that dielectric film 104 made of silicon oxide may improve the TCF of acoustic wave device 101. Conventional acoustic wave device 101 may provide electrode fingers 103 with corrosion.
FIG. 23A is a schematic cross-sectional view of another conventional acoustic wave device 601 disclosed in International Publication No. 2012/102131. Acoustic wave device 601 includes piezoelectric substrate 604, IDT electrodes 602A, and dielectric film 603. IDT electrode 602A is formed on an upper surface of substrate 604, and includes electrode fingers 602 that excite a Rayleigh wave as a main acoustic wave having a wavelength λ. Dielectric film 603 is provided on the upper surface of piezoelectric substrate 604 for covering IDT electrode 602A, and is formed by a sputtering method or a CVD method.
FIG. 23B is an enlarged cross-sectional view of electrode finger 602. Electrode finger 602 includes upper layer 606 and lower layer 605. Angle 607 formed by a side surface and a bottom surface of lower layer 605 is about 90 degrees.
FIG. 23C is an enlarged cross-sectional view of another electrode finger 608 of IDT electrode 602A of acoustic wave device 601. Electrode finger 608 includes upper layer 611 and lower layer 612. Angle 609 formed by a side surface and a bottom surface of lower layer 612 is an acute angle. In other words, electrode finger 608 having a tapered shape.
The shapes of electrode fingers 602 and 608 are formed by a lift-off method employing a photolithography technique, or by an etching method employing a micro-process technique. Conventional acoustic wave device 601 may have a small reflectivity.